In states of injury or disease, chondrocytes alter their pattern of gene expression in response to changes in their surrounding matrix, differences in the mechanical properties of the tissue, changes -in-lbe levels of growth factors and cytokines, and mutations within their genes. In order to understand the mechanisms that lead to these altered patterns of gene expression we have undertaken the approach of gene expression profiling of human chondrocytes. This strategy has led to the identification of two genes, TWIST and HIF-1 that are potentially important for chondrocyte function and pathophysiology. TWIST is a basic helix-loop-helix (bHLH) transcription factor that has been implicated as having an important role in mesodermal cell differentiation. HIF-1a is a helix-loop-helix-PAS (Per, ARNT, Sim) domain containing transcription factor that, along with HIF-1p (also known as ARNT, aryl hydrocarbon receptor nuclear translocator, and also a helix-loop-helix-PAS factor), form the heterodimeric factor HIF-1 that regulates numerous genes (i.e. genes involved in anerobic metabolism, oxygen transport, vasodilation, and blood vessel formation) in response to hypoxia, heavy metals, insulin, insulin-like growth factors and cytokines. The hypothesis to be tested in this application is that the transcription factors TWIST and/or HIF-1(x are involved in regulating the expression of the chondrocyte-specific phenotype. Therefore, in order to learn more about the function of these genes in chondrocytes, we intend to study their function by 1) investigating the expression patterns of TWIST and HIF-1 a in normal, adult and fetal human articular cartilage and cartilage obtained from patients with osteoarthritis, 2) determining the effect of inhibition of, and over-expression of HIF-1 a and TWIST on the chondrocyte-specific phenotype and on chondrogenesis in vitro, 3) identifying genes that are downstream targets of TWIST and HIF-1 a in chondrocytes and, 4) investigating the role Of 02 tension and Hlf-1(x in chondrocyte re-differentiation and chondrogenesis in vitro.